Use default configuration when the one given is empty

Which happens for example when you have solid queue configuration for an
environment but not for others, so you can't find the `workers` and
`dispatchers` keys.

lib/solid_queue/configuration.rb

@@ -23,6 +23,11 @@ def instantiate
       recurring_tasks: []
     }
 
+    DEFAULT_CONFIG = {
+      workers: [ WORKER_DEFAULTS ],
+      dispatchers: [ DISPATCHER_DEFAULTS ]
+    }
+
     def initialize(mode: :fork, load_from: nil)
       @mode = mode.to_s.inquiry
       @raw_config = config_from(load_from)
@@ -61,22 +66,27 @@ def dispatchers
       end
 
       def config_from(file_or_hash, env: Rails.env)
-        config = load_config_from(file_or_hash)
-        config[env.to_sym] ? config[env.to_sym] : config
+        load_config_from(file_or_hash).then do |config|
+          config = config[env.to_sym] ? config[env.to_sym] : config
+          if (config.keys & DEFAULT_CONFIG.keys).any? then config
+          else
+            DEFAULT_CONFIG
+          end
+        end
       end
 
       def workers_options
-        @workers_options ||= options_from_raw_config(:workers, WORKER_DEFAULTS)
+        @workers_options ||= options_from_raw_config(:workers)
           .map { |options| options.dup.symbolize_keys }
       end
 
       def dispatchers_options
-        @dispatchers_options ||= options_from_raw_config(:dispatchers, DISPATCHER_DEFAULTS)
+        @dispatchers_options ||= options_from_raw_config(:dispatchers)
           .map { |options| options.dup.symbolize_keys }
       end
 
-      def options_from_raw_config(key, defaults)
-        raw_config.empty? ? [ defaults ] : Array(raw_config[key])
+      def options_from_raw_config(key)
+        Array(raw_config[key])
       end
 
       def parse_recurring_tasks(tasks)

test/unit/configuration_test.rb

@@ -11,6 +11,14 @@ class ConfigurationTest < ActiveSupport::TestCase
     assert_processes configuration, :dispatcher, 1, batch_size: SolidQueue::Configuration::DISPATCHER_DEFAULTS[:batch_size]
   end
 
+  test "default configuration when config given doesn't include any configuration" do
+    configuration = SolidQueue::Configuration.new(load_from: { random_wrong_key: :random_value })
+
+    assert_equal 2, configuration.configured_processes.count
+    assert_processes configuration, :worker, 1, queues: [ "*" ]
+    assert_processes configuration, :dispatcher, 1, batch_size: SolidQueue::Configuration::DISPATCHER_DEFAULTS[:batch_size]
+  end
+
   test "read configuration from default file" do
     configuration = SolidQueue::Configuration.new
     assert 3, configuration.configured_processes.count